From the handling of large volumes of bulk materials in industry to the measurement of minute amounts of species in the laboratory, there has always been a need to measure flow. Although many design variations are observed, flow apparatus for the measurement of most materials today have been constructed using essentially two principles. These are: (1). the measurement of pressure differential across a restriction in a conduit and (2). the measurement of rotational speed of a propeller primarily in an apparatus commonly known as the turbine meter. Because of the limitations of these two approaches, highly specialized and expensive flow rate apparatus have been developed to meet the special needs of particular flow measurement requirements. These specialized instruments include, for example, employment of Doppler shift measurements, apparatus using radioactive sources, the coupling with hydrogen atoms in nuclear magnetic resonance measurements employing phase locked loop detection techniques, deflection of a curved tube through which materials pass imparting a measurable force tending to straighten the tube, vortex shedding methods and the measurement of a magnet induced electric field. The induction flow meter has been employed for measurement of electrically conductive liquids as far back as 1917.
Thermal techniques have been previously employed to measure flow. The hot wire fluid/air velocity transducer consists basically of an electrically heated fine wire which is immersed in the stream and cooled by the flow. The associated resistance change is detected and provides a measure of flow. Another method is the injection of a small amount of heat into the stream and measuring the time-of-flight between two fixed points in the stream. Still others employ massive and costly machined parts effectively serving as an environmental heat sink and a heater to measure flow.
Systems employing a line restriction, a shaped venturi tube section or orifice plate and the associated measurement of pressure differential across the restriction haved the limitation that the stream is restricted. Materials having high viscosity, pastes and pneumatically pumped solids cannot be effectively measured by this method. The turbine meter is not effective for small flow rates, it requires a seal around a rotary shaft and bearings, it has the same flowing material restrictions as the previous techniques, it results in error or failure when subjected to varying temperature environments and induces vibration or pulsation into the stream. The remaining flow apparatus types are limited by being of the immersion type, not applicable to virtually all materials, delicate, not temperature compensated, require uniform and homogeneous flowing liquids or gases or are costly to manufacture.
Current needs for low cost, high reliability flow indicators having the application flexibility of the instant invention are demanding. To the present, there has been no flow indicator device meeting the stringent, cost effective demands placed on it by the automotive industry for monitoring fuel flow. Another application unsatisfied by prior art devices is the measurement of corrosive materials flow. Further, there is no available instrument to measure pneumatically pumped powdered coal in a fluidized bed or primary heating equipment. Still another heretofore unsatisfied application is the measurement of other pneumatically pumped solids such as acrylic particles in the plastics industry and metal/gas mixtures in the flame spray and plasma deposition industry. Another unsatisfied application resides in the measurement of flow of thick pastes such as slip materials in the ceramic industry. Therefore, a system and device is needed which overcomes the limitations of high cost and inadequacy of prior art of those earlier devices employing components and arrangements subject to high failure rate and high initial and maintenance costs.